Users who are still waiting on the OTA 4.2 update, try this..

Don’t laugh and try these steps:

1) Put your Nexus phone/tablet into Airplane mode.
2) Go to Settings → Apps then swipe to the All tab.
3) Scroll down to the “Google Service Framework”, Force Stop it then hit Clear Data.
4) Go back and disable Airplane mode.
5) Go to Settings → About Phone → System Updates and try again.

I was trying the same steps (minus the ones relating to Airplane mode) last night on my Nexus 7, no success after repeating 5 times. Then I got it on first try with the steps as listed above.

Please report results.

Thanks and gold luck.

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UPDATE: It seems there's a chance of breaking your Google account sync if OTA was forced this way. Try only clearing data of the process "Google Framework Service" first without force closing it. Either way, TRY AT YOUR OWN RISK!